Phosphate tightly associated with a molecule, e.g., a protein, has been known since the late nineteenth century. Since then, a variety of covalent linkages of phosphate to proteins have been found. The most common involve esterification of phosphate to serine, threonine, and tyrosine with smaller amounts being linked to lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine. The occurrence of phosphorylated molecules, e.g., proteins, implies the existence of one or more kinases, e.g., protein kinases, capable of phosphorylating various molecules, e.g., amino acid residues on proteins, and also of phosphatases, e.g., protein phosphatases, capable of hydrolyzing various phosphorylated molecules, e.g., phosphorylated amino acid residues on proteins.
Protein kinases play critical roles in the regulation of biochemical and morphological changes associated with cellular growth and division (D'Urso et al. (1990) Science 250:786-791; Birchmeier et al. (1993) Bioessays 15:185-189). They serve as growth factor receptors and signal transducers and have been implicated in cellular transformation and malignancy (Hunter et al. (1992) Cell 70:375-387; Posada et al. (1992) Mol. Biol. Cell 3:583-592; Hunter et al. (1994) Cell 79:573-582). For example, protein kinases have been shown to participate in the transmission of signals from growth-factor receptors (Sturgill et al. (1988) Nature 344:715-718; Gomez et al. (1991) Nature 353:170-173), control of entry of cells into mitosis (Nurse (1990) Nature 344:503-508; Maller (1991) Curr. Opin. Cell Biol. 3:269-275) and regulation of actin bundling (Husain-Chishti et al. (1988) Nature 334:718-721).
Protein kinases can be divided into different groups based on either amino acid sequence similarity or specificity for either serine/threonine or tyrosine residues. A small number of dual-specificity kinases have also been described. Within the broad classification, kinases can be further subdivided into families whose members share a higher degree of catalytic domain amino acid sequence identity and also have similar biochemical properties. Most protein kinase family members also share structural features outside the kinase domain that reflect their particular cellular roles. These include regulatory domains that control kinase activity or interaction with other proteins (Hanks et al. (1988) Science 241:42-52).
Extracellular-signal-regulated kinases/microtubule-associated protein kinases (Erk\MAPKs) and cyclin-directed kinases (Cdks) represent two large families of serine-threonine kinases (see Songyang et al., (1996) Mol. Cell. Biol. 16: 6486-6493). Both types of kinases function in cell growth, cell division, and cell differentiation, in response to extracellular stimulae. The Erk\MAPK family members are critical participants in intracellular signaling pathways. Upstream activators as well as the Erk\MAPK components are phosphorylated following contact of cells with growth factors or hormones or after cellular stressors, for example, heat, ultraviolet light, and inflammatory cytokines. These kinases transport messages that have been relayed from the plasma membrane to the cytoplasm by upstream kinases into the nucleus where they phosphorylate transcription factors and effect gene transcription modulation (Karin et al., (1995) Curr. Biol. 5: 747-757). Substrates of the Erk\MAPK family include c-fos, c-jun, APF2, and ETS family members Elk1, Sap1a, and c-Ets-1 (cited in Brott et al., (1998) Proc. Natl. Acad. Sci. USA 95, 963-968).
Cdks regulate transitions between successive stages of the cell cycle. The activity of these molecules is controlled by phosphorylation events and by association with cyclin. Cdk activity is negatively regulated by the association of small inhibitory molecules (Dynlacht, (1997) Nature 389:148-152). Cdk targets include various transcriptional activators such as p110Rb, p107 and transcription factors, such as p53, E2F and RNA polymerase II, as well as various cytoskeletal proteins and cytoplasmic signaling proteins (cited in Brott et al., above).
A protein has been isolated in Drosophilia, designated nemo, which has homology to Erk\MAPKs and Cdks. A mammalian homologue of nemo, designated NLK, has been reported (Brott et al., above). This protein kinase autophosphorylates and localizes to a great extent in the nucleus. This protein showed homology to both families of kinases (Erk\MAPKs and Cdks). It did not possess the characteristic MAPK phosphorylation motif TXY in the conserved kinase domain VIII. It instead exhibited the sequence TQE resembling the THE sequence found in some Cdks.
More recently, it was shown that NLK could down-regulate HMG-domain-containing proteins related to POP-1. The signaling protein Wnt regulates transcription factors containing high-mobility group (HMG) domains to direct decisions on cell fate during animal development. In C. elegans, the HMG-domain-containing repressor POP-1 distinguishes the fate of anterior daughter cells from posterior daughter cells throughout development. Wnt signaling down-regulates POP-1 activity in posterior daughter cells, for example, E. Meneghini et al., (1999) Nature 399: 793-797, show that the genes MOM-4 and LIT-1 were also required to down-regulate POP-1 not only in E but in other posterior daughter cells. MOM-4 and LIT-1 are homologous to the mammalian components of the mitogen-activated protein kinase (MAPK) pathway of TAK-1 (transforming growth factor beta activated kinase (and NLK) nemo-like kinase, respectively. MOM-4 and TAK-1 bind related proteins that promote their kinase activity. The authors of the report concluded that a MAPK-related pathway cooperates with Wnt signal transduction to down-regulate POP-1 activity.
In a further report by the same group (Ishitani et al, (1999) Nature 399: 798-802), it was shown that the TAK-1-NLK-MAPK-related pathway antagonizes signaling between beta-catenin and transcription factor TCF. The Wnt-signaling pathway regulates developmental processes through a complex of beta-catenin and the T-cell factor/lymphoid enhancer factor (TCF\LEF) family of high-mobility group transcription factors. Wnt stabilizes beta-catenin which then binds to TCF and activates gene transcription. This signal pathway is conserved in vertebrates, Drosophilia and C. elegans. In C. elegans, MOM-4 and LIT-1 regulate Wnt signaling during embryogenesis. MOM-4 is homologous to TAK-1 (a kinase activated by transforming growth factor beta). LIT-1 is homologous to mitogen-activated protein kinase kinase kinase (MAP3K) and MAP kinase (MAPK)-related NEMO-like kinase (NLK) in mammalian cells. This raised the possibility that TAK-1 and NLK were involved in Wnt signaling in mammalian cells. The authors reported that TAK-1 activation stimulates NLK activity and down-regulates transcriptional activation mediated by beta-catenin and TCF. Injection of NLK suppressed the induction of axis duplication by microinjected beta-catenin in Xenopus embryos. NLK was shown to phosphorylate TCF\LEF factors and inhibit the interaction of the beta-catenin-TCF complex with DNA. Accordingly, the TAK-1-NLK-MAPK-like pathway was shown to negatively regulate the Wnt signaling pathway.
Protein kinases play critical roles in cellular growth. Therefore, novel protein kinase polynucleotides and proteins are useful for modulating cellular growth, differentiation and/or development.